Are humans next after 2 races ??

• James Meek, science correspondent
• The Guardian
• Friday February 23 2006

In a discovery with chilling implications for humans' dominance of the Earth, scientists have discovered that a massive comet or asteroid, similar to the one which killed off the dinosaurs, wiped out the giant reptiles' predecessors 200 million years earlier.

The finding suggests that regular collisions between our planet and large objects act as an evolutionary dice-shaker, sending dominant species back to the starting square and advancing obscure species to dominance. We could be next.

Students of the fossil record have long pondered the greatest catastrophe ever to strike life on Earth: the "Great Dying", 251m years ago, when suddenly, 90% of all marine animals and 70% of land vertebrates perished.

The impact of a comet or asteroid similar to that which brought doom to the dinosaurs 65m years ago was suspected, but the latest edition of the journal Science reports on the first evidence that such a collision actually happened.

Scientists at the universities of Washington and Rochester in the US have found that molecules of helium and argon gas, locked since the Great Dying in carbon, are present in proportions which could only have come from space. As in the later impact, the strike came from a body between six and 12 kilometres wide (four to eight miles).

Researcher Robert Poreda said yesterday the effect would have been to release energy 1m times greater than the biggest earthquake of the past century.

The crash coincided with volcanic activity on an unimaginable scale in what is now Siberia. Over a million years - a relatively short time in geological terms - 1.6m cubic kilometres of lava poured out of the ground, covering the entire planet in a layer 10 metres thick.

"It was the proverbial blast from the double-barrelled shotgun," said Dr Poreda. "We're not sure of all the environmental consequences, but with the impact and the volcanic activity, we do know that Earth was not a happy place.

"It may be that the combined effects of impact and volcanism are necessary to cause such a tremendous extinction."

The nemesis of the dinosaurs also coincided with a gush of lava from below the Earth's crust, in what is now India. In both cases, the impact of the comet or asteroid may have caused the eruption.

Alternatively, the coincidence of the two events could have meant the end for the dinosaurs and their predecessors.

"There has been lots of flood basalt volcanism over time, and many impacts, but these impact events caused major extinctions. Both coincided with periods of heavy volcanic activity," said Dr Poreda.

The dinosaur-killing impact has been tied to a crater on the Yucatan peninsula in Mexico, but it is not known where the earlier collision happened. At the time all Earth's land made up a single continent, Pangea.

The age immediately prior to the dinosaurs was called the Permian. Although there were amphibious reptiles, early versions of the dinosaurs, the dominant life form was the trilobite, visually somewhere between a wood louse and an armadillo.

In their heyday there were 15,000 kinds of trilobite. Around the time of the impact, they disappeared. Something similar happened to the dinosaurs 65m years ago. In the same way that the earlier impact seems to have cleared the way for them, the later crash made room for our ancestors, the mammals.

"The two extinctions are like bookends for the age of the dinosaurs," said Dr Poreda. "The first boundary helped usher in the age of the dinosaurs and the second snuffed it out."

Although it is likely to be millions of years before the next big asteroid or comet impact, there is a growing call for a defence system to be put in place.